Plasma lamps (such as high intensity discharge (HID) lamps and fluorescent lamps) provide extremely bright, broadband light. Plasma lamps are useful in applications such as projection systems, industrial processing, and general industrial and commercial illumination. Typical plasma lamps contain a mixture of a noble gas (such as Argon) and trace substances (such as metal halide salt or mercury) that are excited to form a plasma. Interaction between the ionized noble gas and the trace substance gives rise to light in the ultraviolet (UV), visible, and near infrared spectrums. Gas ionization resulting in plasma formation is accomplished by applying a high voltage across electrodes; these electrodes are contained within the vessel that serves as the reservoir of the gas fill. However, this arrangement suffers from electrode deterioration due to sputtering of the metal electrodes, and therefore exhibits a limited lifetime. In addition, the presence of metal electrodes inside the gas-fill vessel limits the range of noble gas and metal halide salt that can be used.
Electrodeless plasma lamps driven by microwave sources have been disclosed in prior art for more reliable longer lasting lamps. Various methods have been disclosed to couple radio-frequency (RF) energy into the bulb to ionize the gas without the use of any electrodes inside the bulb (vessel). U.S. Pat. No. 2,624,858 (issued to Greenlee, et al.) and U.S. Pat. No. 6,858,985 B2 (issued to Kraus et al.) disclose capacitively coupled electrodeless lamps. FIG. 1 shows an example of the prior art, which is an electrodeless lamp 100 with capacitive coupling. The electrodeless lamp comprises an enclosed glass tube or quartz tube 104 (gas-fill vessel) filled with an inert gas (Argon, etc.) and metal halide salt material (Selenium, etc.). The quartz tube has an outer diameter between approximately 4 millimeters (mm) and 100 mm, and a length of approximately 6 mm and 500 mm. External to the bulb at both ends are two coupling-in structures 102 and 102′ made from highly conductive metal (such as copper with an added thin layer of a high a melting-point metal or a thin layer of dielectric to act as a diffusion barrier between copper and quartz) that applies the RF field to the bulb using capacitive coupling. Depending on the area of the coupling-in structures 102 and 102′ and the size of the bulb, the RF source can have a frequency range approximately between 1 megahertz (MHz) and 10 GHz for optimum coupling of the RF energy into the bulb. The RF energy ionizes the gas inside the bulb 104 and vaporizes/melts the salt material. The interaction between the ionized gas and salt vapor produces an intense source of light 106. Depending on the inert gas and salt material, different emission spectra can be produced from the lamp 100.
The prior art shown in FIG. 1 consist of a quartz bulb (or a tube of glass) filled with a noble gas and metal halide salt material (Selenium or other). Electrodes, external to the bulb, apply a high energy RF field to the gas to ionize it. The ionized gas will in turn heat the salt material to melt/vaporize it. Interaction between the ionized noble gas and the salt vapor results in high intensity illumination from the bulb. Because electrodes are external to the bulb, these types of bulbs do not have the reliability issues associated with electrode degradation as a result of exposure to plasma and salt material in conventional plasma lamps. The coupling efficiency of the RF to the plasma, which is a critical parameter for overall efficiency of the lamp, depends on the impedance of the capacitive coupling-in structures. This impedance depends inversely on the frequency of the RF source and the coupling capacitance of the external electrodes. For a number of applications it is desirable to use a lower frequency RF source, particularly to lower the cost of the lamp. However, the size of the bulb and the electrodes can limit the frequency of the RF source to frequencies in the low gigahertz (GHz) range. In addition, for longer bulbs the separation between coupling-in electrodes will increase, reducing the strength of the electric field. Thus, for this type of electrodeless lamp the number of design parameters to optimize the performance and cost of the lamp is limited.
Microwave discharge type electrodeless lamps have been disclosed in U.S. Pat. No. 6,617,806B2 (issued to Kirkpatrick et al.) and U.S. Pat. No. 6,737,809B2 (issued to Espiau et al.). These inventions disclose similarly basic configurations of a gas fill encased in either a bulb or a sealed recess within a dielectric body to form a waveguide or a resonator. Microwave energy from a source, such as a magnetron or a microwave solid state power amplifier, is introduced into the waveguide. The microwave energy is then coupled into the bulb to heat the plasma and metal halide salt material. The prior art disclosed in U.S. Pat. No. 6,737,809B2 is shown in FIG. 2. FIG. 2 is a schematic of another example of prior art: a microwave discharge electrodeless lamp 200. A gas-fill vessel (bulb) 206 made from quartz is filled with an inert gas (Argon, etc.) and salt material (Selenium, etc.). The bulb is placed inside an opaque dielectric waveguide 202 (resonator/cavity) with only the tip of the bulb being outside the dielectric. The dielectric waveguide 202 is made from a higher dielectric constant material (such as alumina) compared to quartz. The size of the dielectric waveguide 202 is comparable to the wavelength of the RF frequency source exciting the plasma. The typical diameter of the dielectric waveguide 202 is about half the wavelength (inside the dielectric waveguide 202) of the RF source. The RF source is coupled into the dielectric using an RF probe 204 and it is coupled out using RF probe 204′ through the back of the dielectric waveguide. The dielectric waveguide 202 couples the RF into the bulb 206 to ionize the gas and melt/vaporize the salt. The interaction between the ionized gas and salt vapor causes light emission 208 from the bulb 206. The light is only emitted from the top of the lamp 200 only since it is surrounded with opaque dielectric 202 on the sides. In some cases, the dielectric wall surrounding the bulb 206 is coated with a reflective surface to increase the amount of light that is harvested from the lamp 200.
FIG. 3 is a schematic of the prior art, shown as a cross-section of the lamp of FIG. 2, with the cross-section being taken through approximately the middle of the lamp 200. In this example, the dielectric waveguide (resonator/cavity) 202 surrounds the gas-fill vessel (bulb) 206 coupling RF energy to create plasma inside the bulb causing light emission 208 from the bulb 206.
Most of the quartz bulb, except for a small portion of the tip, is enclosed within an opaque dielectric waveguide. RF energy is applied to the dielectric waveguide (or resonator) through the back of the dielectric via an RF probe. The light is “harvested” from the top of the dielectric. To operate the lamp at lower frequencies and achieve the same coupling efficiency achieved at higher frequencies, the size of the dielectric waveguide/resonator has to be increased. However, in a number of applications the overall size of the lamp that can be used is limited; so the size of the dielectric—and therefore the frequency of operation—will be limited as well. In addition, scaling the lamp to get a higher light output power is difficult with this design.
The above-described prior art and their associated problems clearly demonstrate a need for an electrodeless plasma lamp that is efficient, robust, and easily scalable to both different sizes and different frequency ranges of RF sources.